Rotational speed sensor

ABSTRACT

Disclosed is a rotational speed sensor comprising a rotatable ring, e.g. connectable to a bearing. The rotational speed sensor has K magnetic pole pairs distributed angularly over the rotatable ring. Also, sensor means are present positioned relative to the rotatable ring such that a varying magnetic field is detected by the sensor means. The sensor means comprise a first pair of magnetic sensors, the first pair of magnetic sensors being positioned 2πL/K radians apart from each other. The sensor means may further comprise a second pair of magnetic sensors, the second pair of sensors being positioned 2πM/K radians apart from each other. The first pair of sensors and second pair of sensors are positioned at a relative position of (2π/K)*((2n−1)/2) radians.

The present invention relates to a rotational speed sensor comprising arotatable ring, e.g. connectable to a bearing, having K magnetic polepairs distributed angularly over the rotatable ring, K being an integergreater than one, and sensor means positioned relative to the rotatablering such that a varying magnetic field is detected by the sensor means.

Such a rotation sensor is known from American patent U.S. Pat. No.5,184,069, which describes a rotation sensor for detecting relativerotation between two components which are coupled by an anti-frictionbearing. The rotation sensor comprises a tone ring with a layer ofmagnetic ink, the layer defining multiple magnetic poles, with the northand south poles being alternatively positioned at the circumference ofthe ring. The rotation sensor further comprises a transducer fordetecting a varying magnetic field when the two components rotate withrespect to each other.

However, the arrangement of U.S. Pat. No. 5,184,069 is susceptible to anumber of error mechanisms. The layer having multiple magnetic poles isdifficult to manufacture within very strict tolerances. The distancebetween north and south oriented magnetic poles on the layer is notalways constant over the entire layer circumference. This causes thedetected magnetic field to have anomalies when the disc rotates, causedby the phase errors, also indicated by the term jitter.

Further problems occur when the layer on which the magnetic poles arearranged is not making a perfect circular motion. This may be caused byradial movement of the layer with respect to the magnetic sensor, andcauses further errors in the sensor output signal.

Also, external magnetic fields may influence the signal generated by themagnetic sensor.

The present invention seeks to provide a rotational speed sensor havingan improved performance, especially with respect to jitter.

This is accomplished according to the invention by a rotational speedsensor according to the preamble defined above, in which the sensormeans comprise at least a first pair of magnetic sensors, the first pairof magnetic sensors being positioned 2πL/K radians apart from eachother, L being an integer between 1 and K−1. In such a configuration,the two magnetic sensors of the first pair look at the same pole ofdifferent magnetic dipoles. This allows to obtain a signal with a highersignal strength, reducing the effect of jitter.

In a further embodiment, L is equal to K/2, i.e. the magnetic sensors ofthe first pair are positioned diametrically opposite to each other. Thisembodiment allows for a reduced sensitivity to jitter, but also areduced sensitivity to movement of the disc in a direction along theline connecting the two magnetic sensors, i.e. radial movement of thedisc.

The sensor means may in a further embodiment comprise at least onesecond pair of magnetic sensors, the second pair of sensors beingpositioned 2πM/K radians apart from each other, M being an integerbetween 1 and K−1, the first pair of sensors and second pair of sensorsbeing positioned at a relative position of (2π/K)*((2n−1)/2) radians, nbeing an integer greater than one. In this configuration, the secondpair of sensors looks at the opposing pole of the magnetic dipoles, i.e.in anti-phase with the first pair of magnetic sensors. This allows tocancel out external influences, such as external magnetic fields andtemperature effects.

To allow detection of the direction of rotation, the sensor means mayfurther comprise an additional magnetic sensor, positioned at(2π/K)*((2m−1)/4) radians from the first or second pair of magneticsensors, m being an integer greater than one. From the phase of theadditional magnetic sensor signal, compared with the phase of the othermagnetic sensors, the direction of rotation may be determined. Dependingon the configuration, a phase advance may indicate a clockwise orcounter clockwise rotation.

One further embodiment comprises magnetic sensors of the Hall effecttype. These kind of sensors allow to operate in a high temperatureenvironment.

In further embodiments, the rotational speed sensor is connectable tosignal processing means. The signal processing means may be arranged toadd the signals from the magnetic sensors of the first pair to obtain afirst sensor pair signal. Also, the signal processing means may bearranged to add the signals from the magnetic sensors of the first pairto obtain a first sensor pair signal and to add the signals from themagnetic sensors of the second pair to obtain a second sensor pairsignal and to subsequently subtract the second pair signal from thefirst pair signal. Furthermore, the signal processing means may bearranged to add the signals from the magnetic sensors of the first pairand/or the second pair to obtain a first sensor pair signal and/or asecond sensor pair signal, respectively, and the signal processing meansmay be further arranged for determining a speed direction from the firstsensor pair signal and/or the second pair signal and the signal from theadditional magnetic sensor. These speed sensors with processing meanselements may provide the above described advantages resulting inelectrical signals for further processing or control purposes. Moreadvantageously, the sensor means and signal processing means areintegrated, e.g. in a bearing. As signal lines from the sensors to theprocessing elements will then be very short, the output signal will bevery resistant to external influences, such as electromagneticinterference.

The present invention will now be explained in further detail using anumber of exemplary embodiments, with reference to the accompanyingdrawings, in which

FIG. 1 shows a top view of a rotational speed sensor according to oneembodiment of the present invention;

FIG. 2 shows a block diagram of the signal processing means associatedwith the rotational speed sensor of the present invention;

FIG. 3 shows a block diagram of further signal processing meanselements.

An exemplary embodiment of the rotational speed sensor 10 is shown in atop view in FIG. 1. The speed sensor 10 comprises a ring or disc 11,which may be attached to a rotating element, e.g. a bearing ring. Thedirection of rotation is indicated in FIG. 1 by the arrow. The ring 11comprises K magnetic dipoles 12, in the configuration shown K=12. Itwill be clear that a smaller or larger number of magnetic dipoles 12 maybe present. Each magnetic dipole 12 comprises a south pole 13 and anorth pole 14. The magnetic dipole orientation is such that at thecircumference of the disc 11, the south pole and north pole alternate.Each magnetic dipole 12 subtends an angle α of the disc 11. Ideally,this angle a would be the same for each magnetic dipole 12 of the speedsensor 10. Also, the dimension of south pole 13 and north pole 14 wouldbe identical. However, in practice, the dimensions of the south pole 13and north pole 14, and of the magnetic dipoles 12 mutually will beslightly different, mainly due to fabrication tolerances. This willcause variations in the magnetic field at a predetermined position atthe circumference of the disc 11, an effect also indicated by the termjitter.

Also, when using only a single magnetic sensor to detect the magneticfield at the predetermined position, variations in the distance betweensensor and disc will cause anomalies in the detected signal. Externalmagnetic fields will also negatively influence the sensor signal.

In the embodiment shown in FIG. 1, the speed sensor 10 comprises 5magnetic sensors 15-19 at the circumference of the disc 11. The magneticsensors 15-19 may be attached to a fixed part, e.g. a fixed ring of abearing. The magnetic sensors 15-19 may then be used to detect therotational speed of the disc 11 relative to the magnetic sensors 15-19.

A first pair of magnetic sensors is formed by the sensors 15 and 16.These sensors are positioned exactly opposite each other (π radians) andsense the same polarization of oppositely positioned magnetic dipoles12. The sensors 15 and 16 each provide a sinusoidal shaped signal. Inmore generalized terms, the sensors 15, 16 of the first magnetic sensorpair must look at the some polarization, or they must be positioned atan angle of 2πL/K radians apart, in which K is the number of magneticdipoles 12 of the sensor 10, and L is an integer between 1 and K−1.

A rotational speed sensor 10 equipped with only the first pair ofmagnetic sensors 15, 16 will show an improved jitter behavior. Thesignals from the first pair of magnetic sensors 15, 16 may be added,providing a sinusoidal signal with double the amplitude as compared to asingle sensor. Also, small errors caused by jitter will be smoothed,thus leading to a jitter-improved signal.

When the magnetic sensors 15, 16 of the first pair are positionedexactly at π radians from each other, the speed sensor 10 will also bemore resistant to movement of the disc 11 along the line between the twomagnetic sensors 15, 16. When the disc 11 moves towards the sensor 15,the signal delivered by that sensor 15 will become larger, but at thesame time, the signal delivered by the other sensor 16 will becomesmaller. By adding the two signals, the resulting signal will show no orless anomalies. Further first pairs of magnetic sensors may be added atmultiple angles of the angle α (or at positions equal to 2π*1/K, 1 beingequal to a value between 1 and K−1), in which the magnetic sensors ofthe further first pair are also positioned at π radians from each other,to provide further axes along which the sensitivity to radial motion ofthe disc 11 is reduced.

To further improve the behavior of the rotational speed sensor 10, afurther pair of sensors 17, 18 may be added, which ‘look’ at the otherpole of the magnetic dipoles 12. In the top view shown in FIG. 1, themagnetic sensors 17, 18 of the second pair look at a transition from asouth pole to a north pole, while the magnetic sensors 15, 16 look at atransition from a north pole to a south pole, i.e. the first pair andsecond pair are in anti-phase. The signals from the second pair 17, 18may also be added (as for the first pair), and the resulting signals(which already provide a better jitter resistance) may be subtractedfrom each other to provide an even better jitter resistant signal. Also,the susceptibility to radial movement of the disc 11 is improved in thesame manner as in the embodiment described above. As the resulting sinewave is of a better quality, it will be possible to obtain a moreaccurate interpolation of the signal.

In FIG. 2, a schematic block diagram is shown of processing means thatmay be connected to the rotational speed sensor 10 for providing a speedsignal. The processing means comprise a first addition element 20, foradding the signals A and B from the magnetic sensors 15 and 16 of thefirst pair. Furthermore, the processing means comprise a second additionelement 21 for adding the signals C and D from the magnetic sensors 17,18 from the second pair. The resulting signals are subtracted from eachother in subtraction element 22. At the output of the subtractionelement 22, a signal Vout is present, which has an amplitude which isthe quadruple of a single magnetic sensor. Furthermore, the signal has areduced sensitivity against jitter and radial movement of the disc 11.

In more general terms, the second pair of magnetic sensors 17, 18 shouldbe positioned at an angular distance of (2π/K)*((2n−1)/2) radians, nbeing an integer greater than one. As in the first pair, the magneticsensors 17 and 18 of the second pair should be positioned relative toeach other at an angular distance of 2πM/K, in which M is an integerbetween one and K−1.

A further advantage of the present rotational speed sensor 10 is that anexternal magnetic field has the same influence on the signal of thefirst pair of magnetic sensors 15, 16 as on the second pair of magneticsensors 17, 18. However, as the signals from these sensors aresubtracted from each other, the external influence contribution cancelsout. Thus, the present rotational speed sensor 10 has a betterresistance against external magnetic field disturbances than prior artsensors, both in static and dynamic conditions.

The rotational speed sensor 10 can easily be modified to allow detectionof the direction of rotation of the disc 11. To this end, the speedsensor 10 is further provided with an additional magnetic sensor 19,which is positioned relative to the other magnetic sensors 15-18 with amultiple of π/2 radians. In more general terms, the additional sensor 19should be positioned at an angular distance of (2π/K)*((2m−1)/4) radiansfrom the first or second pair of magnetic sensors, m being an integergreater than one. The signal from the additional sensor 19, or ratherthe phase of the signal, can than be compared with the signal from oneof the magnetic sensors 15-18. Depending on the relative position of theone sensor and the additional sensor 19, the direction of rotation maybe determined. Also, the signal from the additional sensor 19 may becompared with the sensor speed output signal Vout.

A plurality of first and second pairs of magnetic sensors 15-18 may beprovided to even further reduce the sensitivity to external magneticfields, jitter and radial movements.

FIG. 3 shows a block diagram of a further element of the processingmeans associated with the speed sensor 10. A phase comparator 23compares the phase of the signal of the additional sensor 19 (indicatedwith E) and a signal of the first magnetic sensor 16 (indicated with A).It will be clear that the signal A can also be replaced with the signalVout which is output by the subtraction element 22.

When the magnetic sensors 15-19 are provided as Hall sensors, the speedsensor 10 is able to operate at high operating temperatures, which maybe advantageous in many applications. Also, in the differentialmeasurement variant discussed above, temperature compensation of theHall effect sensors 15-19 will automatically occur.

The speed sensor 10 may be applied in many applications for measurementof rotational speed, e.g. in application where bearings are used. Thedisc 11 is then affixed to one of the rotating parts, while the magneticsensors 15-18 are affixed to the other rotating part (or static part).The processing means 20-23 are advantageously integrated with the speedsensor 10. The resulting short signal leads will even further improvethe resistance against external electromagnetic fields.

1-8. (canceled)
 9. A rotational speed sensor comprising a rotatablering, the rotational speed sensor having K magnetic pole pairsdistributed angularly over the rotatable ring, K being an integergreater than one, and sensor means positioned relative to the rotatablering such that a varying magnetic field is detected by the sensor means,the sensor means comprising at least a first pair of magnetic sensors,the first pair of magnetic sensors being positioned 2πL/K radians apartfrom each other, L being an integer between 1 and K−1, wherein thesensor means comprise at least one second pair of magnetic sensors, thesecond pair of sensors being positioned 2πM/K radians apart from eachother, M being an integer between 1 and K−1, the first pair of sensorsand second pair of sensors being positioned at a relative position of(2π/K)*((2n−1)/2) radians, n being an integer greater than one.
 10. Therotational speed sensor according to claim 9, wherein K is an eveninteger value and L is equal to K/2.
 11. The rotational speed sensoraccording to claim 9, wherein the sensor means further comprise anadditional magnetic sensor, positioned at (2/K)*((2m−1)/4) radians fromthe first or second pair of magnetic sensors, m being an integer greaterthan one.
 12. The rotational speed sensor according to claim 11, whereineach of the first pair of magnetic sensors, the second pair of magneticsensors, and the additional sensor is a Hall type sensor.
 13. Therotational speed sensor according to claim 9, wherein the rotationalspeed sensor is connectable to signal processing means, the signalprocessing means being arranged to add the signals from the first pairof magnetic sensors to obtain a first sensor pair signal.
 14. Therotational speed sensor according to claim 9, wherein the rotationalspeed sensor is connectable to signal processing means, the signalprocessing means being arranged to add the signals from the first pairof magnetic sensors to obtain a first sensor pair signal and to add thesignals from the second pair of magnetic sensors to obtain a secondsensor pair signal and to subsequently subtract the second pair signalfrom the first pair signal.
 15. The rotational speed sensor according toclaim 11, wherein the rotational speed sensor is connectable to signalprocessing means, the signal processing means being arranged to add thesignals from the first pair of magnetic sensors and/or the second pairof magnetic sensors to obtain a first sensor pair signal and/or a secondsensor pair signal, respectively, and the signal processing means arearranged for determining a speed direction from the first sensor pairsignal and/or the second pair signal and the signal from the additionalmagnetic sensor.
 16. The rotational speed sensor according to claim 13,wherein the sensor means and the signal processing means are integrated.17. The rotational speed sensor according to claim 14, wherein thesensor means and the signal processing means are integrated.
 18. Therotational speed sensor according to claim 15, wherein the sensor meansand the signal processing means are integrated.